Myocardial ischemia, a leading cause of mortality, involves oxygen starvation of the myocardium. Myocardial ischemia can lead to myocardial infarction if left untreated. Early detection of myocardial ischemia provides the opportunity for a wide range of effective therapies such as surgical revascularization, neural stimulation, and drug delivery to reduce cardiac workload or improve cardiac circulation. Unfortunately, many episodes of myocardial ischemia do not cause excessive pain or other noticeable warning signs, and often go undetected.
An electrocardiogram (ECG) or electrogram (EGM) presents a PQRST waveform sequence that characterizes the cyclical cardiac activity of a patient. The T-wave can be used to identify an ischemic condition. U.S. Pat. No. 6,016,443 to Ekwall et al., for example, describes an implantable ischemia detector that employs a repolarization sensor and a patient workload sensor to identify ischemic episodes. The repolarization sensor detects T-wave amplitude or duration to identify increased heart rate. The workload sensor detects patient activity such as exercise by monitoring body movement, muscle sounds, fluid pressure waves, or metabolic changes. When the T-wave indicates an increased heart rate, without a corresponding increase in workload, the detector identifies an ischemic condition.
The ST segment, also associated with the repolarization of the ventricles, is typically close in amplitude to the baseline, i.e., isoelectric amplitude, of the signal sensed between consecutive PQRST sequences. During episodes of myocardial ischemia, the ST segment amplitude deviates from the baseline. Accordingly, deviation in the ST segment is often used to identify an occurrence of myocardial ischemia.
U.S. Pat. No. 6,021,350 to Mathson, for example, describes an implantable heart stimulator having an ischemia detector that indicates an ischemic condition based on elevation of the ST-segment above a baseline. Alternatively, the ischemia detector may rely on a measure of heart activity or patient workload. The stimulator controls the rate of stimulation based on the detection of ischemia using either of the alternative detection modes.
Unfortunately, the use of the ST segment as an indicator of ischemia can be unreliable. The ST segment may deviate from the baseline due to other factors, causing false indications of myocardial ischemia. For example, the ST segment may deviate from the baseline due to changes in the overall PQRST complex, possibly caused by axis shifts, electrical noise, cardiac pacing stimuli, drugs and high sinus or tachycardia rates that distort the PQRST complex. Consequently, the reliability of the ST segment as an indicator of myocardial ischemia can be uncertain.
U.S. Pat. No. 6,128,526 to Stadler et al. describes an ischemia detector that observes variation in the ST segment to identify an ischemic condition. To improve reliability, the detector is designed to filter out ST segment variations caused by factors other than ischemia, such as axis shift, electrical noise, cardiac pacing, and distortion in the overall PQRST complex.
Efforts to verify the reliability of the ST segment have generally proven complicated. Accordingly, there continues to be a need for a simplified system capable of automatically and reliably detecting myocardial ischemia.